A basic structure of a frequency spectrum analyzer is shown in FIG. 5. The conventional spectrum analyzer of FIG. 5 includes a frequency mixer 11, a local oscillator 21, an IF (Intermediate Frequency) filter 31, a detector 41, an A/D (Analog-Digital) converter 51, a ramp generator 61, and a signal process and display 81.
The frequency spectrum analyzer of FIG. 5 converts the frequency of an input signal to be analyzed by the frequency mixer 11. The frequency mixer 11 mixes the input signal with a local signal from the local oscillator 21 and produces an IF signal. The local oscillator 21 is driven by a ramp signal from the ramp generator 61 so that the frequency of the local signal varies (sweeps) linearly in response to the ramp signal. The IF filter 31 filters the IF signal to have a predetermined band width. The detector 41 receives the IF signal from the IF filter 31 and produces a DC voltage which is proportional to an AC amplitude of the IF signal. The DC voltage from the detector 41 is converted to a digital signal by the AD converter 51. The digital signal is provided with predetermined data processing by the signal process and display 81 and the result of the data process is displayed on the display screen.
The ramp generator 61 also provides the ramp signal to the AD converter 51 so that the local oscillator 21 and the AD converter 51 interact with each other. The ramp signal from the ramp generator 61 defines a range of the swept frequency of the local oscillator 21, i.e., a span of frequency spectra in the input signal to be measured. The frequency span in this case is a range of frequency shown on a horizonal axis of the display. Thus, when the frequency span has a certain range of frequency relative to the input signal, the frequency spectrum of the input signal is shown on the display as frequency domain data.
When the frequency span is zero, it means that the ramp signal from the ramp generator 61 is set to a fixed voltage so that a frequency of the local signal is fixed and mixed with the input signal. In this situation, what is shown on the horizontal axis of the display is a time domain response of the input signal for a fixed frequency of the local signal. Therefore, a spectrum analyzer is capable of measuring and analyzing the input signal in the frequency domain as well as in the time domain.
The spectrum analyzer may include an external trigger function in which the sweep generator 61 receives a trigger signal from an external source through a trigger input terminal. In this arrangement, the sweep generator 61 generates a ramp signal in synchronism with the trigger signal. Thus, in case where an input signal to be analyzed is a burst signal, by an external trigger signal having a predetermined timing relationship with the start of the burst signal, the spectrum analyzer can effectively analyze the burst signal.
The conventional spectrum analyzer functions as a receiver for measuring an input signal of a single frequency by setting the zero frequency span as noted above. In this setting, the ramp signal from the ramp generator 61 is set to a fixed voltage so that a fixed frequency of the local signal is mixed with the input signal. The local signal frequency is so adjusted that the frequency difference between the local signal and the input signal is always tuned to the center frequency of the IF filter.
An example of application of the zero span mode of the spectrum analyzer is shown in FIGS. 3A wherein signals in a TDMA (Time Division Multiple Access) such as a PHS (Personal Handy Phone System) are illustrated. A reference control channel having a frequency f1 and a communication channel having a frequency f2 are shown in FIG. 3A. In measuring a time difference t1 between the burst signal in the reference control channel and the information A in the communication channel, two receivers (spectrum analyzers), one that is tuned to the frequency f1 to detect a reference timing and the other that is tuned to the frequency f2 to detect the start timing of the information signal, are required.
Further, in the example of FIG. 3A, to synchronize the operation of the two receivers, it is necessary to prepare a trigger extracting circuit to produce a trigger signal from the receiver tuned to the reference channel. The trigger signal is applied to the trigger input terminal of the receiver tuned to the communication channel. Thus, two receivers operate in synchronism with each other.
Another example of measurement application using the zero span mode of the spectrum analyzers is shown in FIG. 4. As shown in a mobile communication system of FIG. 4, a mobile radio transceiver 92 directly receives a transmission signal from a base station 91 through a path 400. The mobile radio transceiver 92 also receives another transmission signal which has been reflected by an object 93 such as a building. To measure a time delay of the reflected transmission signal relative to the direct transmission signal in the situation of FIG. 4, it is necessary to use two receivers (spectrum analyzers), although the frequency in the two signals are the same. One receives the direct transmission signal through a reference antenna and the other receives the reflected transmission signal through a measurement antenna.
As explained in the foregoing, for measuring the time difference between two signals with different frequencies or the time delay between the two signals with the same frequency, two receivers (spectrum analyzers) are necessary. Further, for measuring the time differences between the two signals by the conventional spectrum analyzers, it is necessary to prepare an additional circuit to produce a trigger signal based on the input signal to synchronize the operations between the two spectrum analyzers.